1. Field of the Invention
The invention relates generally to seal assemblies for sealing between a rotating and a static member. In one aspect, and more particularly, the invention relates to seals for rolling cone bits used to drill a borehole for the ultimate recovery of oil, gas or minerals. Still more particularly, the invention relates to energizers for mechanical face, metal-to-metal seals that are employed to seal and protect the bearing surfaces between the rolling cone cutters and the journal shafts on which they rotate.
2. Description of the Related Art
An earth-boring drill bit is typically mounted on the lower end of a drill string. With weight applied to the drill string, the drill string is rotated such that the bit engages the earthen formation and proceeds to form a borehole along a predetermined path toward a target zone.
A typical earth-boring bit includes one or more rotatable cone cutters. The cone cutters roll and slide upon the bottom of the borehole as the drillstring and bit are rotated, the cone cutters thereby engaging and disintegrating the formation material in their path. The rotatable cone cutters may be described as generally conical in shape and are therefore referred to as rolling cones.
Rolling cone bits typically include a bit body with a plurality of journal segment legs. The rolling cones are mounted on bearing pin shafts (also called journal shafts or pins) that extend downwardly and inwardly from the journal segment legs. As the bit is rotated, each cone cutter is caused to rotate on its respective journal shaft as the cone contacts the bottom of the borehole. The borehole is formed as the action of the cone cutters removes chips of formation material (“cuttings” or “drilled solids”), which are carried upward and out of the borehole by the flow of drilling fluid that is pumped downwardly through the drill pipe and out of the bit. Liquid drilling fluid is normally used for oil and gas well drilling, whereas compressed air is generally used as the drilling fluid in mining operations.
Seals are provided in glands formed between the rolling cones and their journal shafts to prevent lubricant from escaping from around the bearing surfaces and to prevent the cutting-laden, abrasive drilling fluid from entering between the cone and the shaft and damaging the bearing surfaces. When cuttings are conveyed into the seal gland, they tend to adhere to the gland and/or seal component surfaces, and may cause deformation, damage and/or slippage of the seal components. Moreover, the cuttings can accelerate abrasive wear of all seal components and of the bearing surfaces.
In oil and gas drilling, the cost of drilling a borehole is proportional to the length of time it takes to drill to the desired depth and location. The time required to drill the well, in turn, is greatly affected by the number of times the drill bit must be changed before reaching the targeted formation. This is the case because each time the drill bit wears out or fails as a bore hole is being drilled, the entire string of drill pipes, which may be miles long, must be retrieved from the borehole, section by section in order to replace the bit. Once the drill string has been retrieved and the new bit installed, the bit must be lowered to the bottom of the borehole on the drill string, which again must be constructed section by section. The amount of time required to make a round trip for replacing a bit is essentially lost from drilling operations. As is thus obvious, this process, known as a “trip” of the drill string, requires considerable time, effort and expense. It is therefore advantageous to maximize the service life of a drill bit. Accordingly, it is always desirable to employ drill bits that will be durable enough to drill for a substantial period of time with acceptable rate of penetration (ROP).
The durability of a bit and the length of time that a drill bit may be employed before it must be changed depend upon numerous factors. Importantly, the seals must function for substantial periods under extremely harsh downhole conditions. The type and effectiveness of the seals greatly impact bit life and thus, are critical to the success of a particular bit design.
One cause of bit failure arises from the severe wear or damage that may occur to the bearings on which the cone cutters are mounted. These bearings can be friction bearings (also referred to as journal bearings) or roller type bearings, and are typically subjected to high drilling loads, high hydrostatic pressures, and high temperatures.
As previously mentioned, the bearing surfaces in typical bits are lubricated, with the lubricant being retained within the bit by use of seals. Certain metal-to-metal seals (sometimes also referred to as mechanical seals or mechanical face seals) have been employed in rolling cone bits. Metal-to-metal seals were developed in order to increase the working life of the bearings, given that the failure of older elastomeric o-ring seals was one of the most frequent causes of bit failure when drilling at deeper depths. However, with metal-to-metal seals, great care and attention must be employed in their design, manufacture and assembly to ensure that, in use, the engaging sealing surfaces remain undamaged and in close contact with one another so as to ensure a good seal.
The mechanical seal typically includes a rigid metal seal ring having a dynamic seal surface, one that is placed into rotating contact against a non-rotating seal surface such as a surface of an adjacent hard metal ring. The seal ring also includes a static seal surface that engages a surface that is stationary with respect to the seal ring. Typically, a resilient energizing ring is disposed against the static seal surface of the seal ring in order to bias the dynamic seal surface of the metal seal ring into firm contact with the adjacent dynamic sealing surface. In such assemblies, the energizing ring or “energizer” does not itself provide the dynamic seal, but instead provides only a static seal, and serves to bias together other components that provide the dynamic seal. Although the bit will experience severe and changing loading, as well as a wide range of different temperature and pressure conditions, the dynamic and static seal surfaces must nevertheless remain sealingly engaged throughout the life of the bit's cutting structure in order to prevent the lubricant from escaping and/or cuttings from entering the lubricated areas.
One of the failure modes for mechanical seals is undesirable energizer rotation. That is, in many designs, it is desirable that the energizer remain stationary with respect to the adjacent seal components that it engages. Unfortunately, once an energizer begins to rotate, it wears away quickly, resulting in loss of lubricant from the bearing surfaces or the ingress of abrasive drilling fluid and/or drilled cuttings into the bearing region since a pressure difference exists on two sides of the energizer. As a result of either or both of these conditions, the mechanical seals will prematurely fail, leading ultimately to bit failure and the need to “trip” the drill string in order to replace the failed bit. Accordingly, protecting the integrity of the seal is of utmost importance.
One known technique intended to prevent the energizer from undesired rotation is to roughen the contact surfaces of the bearing shaft and the seal ring that are engaged by the energizer. This may be done by blasting the region or by special machining. However, each of these techniques is time consuming and thus adds additional cost and time to manufacturing the drill bit. Further, roughening the contact surfaces is not entirely reliable, and in certain instances, it cannot provide sufficient friction to prevent the energizer from rotating. Further still, once some energizer rotation has occurred, the roughened surface abrades the engaged energizer surface, and thereby accelerates the wear and deterioration of the energizer. Thus, the proposed solution to seal failure caused by undesirable energizer rotation may, in certain instances, actually exacerbates the problem and leads to energizer deterioration and seal failure earlier than would otherwise occur.
It is therefore desirable that a new energizer be devised, one providing substantial biasing force to maintain good sealing contact between opposing metal seal rings, and also providing sufficient contact pressure and friction to prevent the energizer from rotating. Preferably, the energizer could be employed anywhere in a seal assembly where the energizer is to remain static with respect to an adjacent, engaged surface. Further still, it would be preferable if the manufacturing and assembly of the seal components were relatively quick and simple to accomplish.
Accordingly, to provide a drill bit with better performance and longer life, and thus to lower the drilling costs incurred in the recovery of oil and other valuable resources, it would be desirable to provide a seal assembly having the potential to provide longer seal life than conventional metal-to-metal seals. Preferably, such seal assemblies would provide a bit that will drill with acceptable ROP for longer periods so as to increase bit life and increase in footage drilled as compared to bits employing conventional seals.